It is well known that a constituent of an image is often desired to be extracted therefrom to be used independently or in combination with other images to produce another image in the field of electronic image reproduction. Practically, for example, when only a commodity image printed with the background must be extracted therefrom to be printed onto a catalogue, an image extraction masking method is adopted for that use. That is by using a conventional method in which by superimposing an image extraction masking film which is composed of a transparent portion corresponding to the image component to be extracted and an opaque portion corresponding to the unnecessary portion onto the original or a reproduction image thereof by means of a photographic technique, a reproduction image eliminated of the unnecessary portion can be obtained.
The background portion of the image in which a desired image is to be framed can also be extracted instead therefrom by using the method. In passing, the method is also capable of producing a negative reproduction image from a positive original.
The conventional image extraction mask film forming process has required manual work. That is, by at first tracing the outline of a determined objective image onto a transparent sheet by means of a kind of tracing method, and then by painting out the inside or the outside of the outline, an image extraction masking film is produced. Because this work require special skill and long time, the conventional image extraction mask producing process has been a main problem in the photomechanical process.
To overcome the above problem, a method by which the unnecessary portion of a reproduction image of an original is painted out with an opaque ink, and a method by which the unnecessary portion is covered with a so-called peel-off film substituting for said ink are disclosed; however, these methods have little effect on reducing time consumed in the masking process.
Japanese Patent Laid Open No. 47-17501 discloses another method, in which an image extraction mask is obtained at first by donating a specific color to the unnecessary portion of an image in taking a color photograph thereof, and then by extracting or erasing the unnecessary portion to which the specific color is donated. Obviously, this method is not suitable for processing colored originals.
In order to resolve the drawbacks of the above-mentioned conventional methods, Japanese Patent Application No. 53-13119 (Laid Open No. 55-57846) discloses a method, in which the outline of a determined objective image electronically detected by means of a sensor composed of photoelectronic materials are afterwards manually traced to produce an image extraction mask. Meanwhile, Japanese Patent Application No. 56-52106 (Laid Open No. 57-165836) discloses a method, in which an image extraction mask is formed according to address data previously obtained using an address designation means such as a digitizer and an image signal obtained by analyzing the image of the original comprising the determined objective image using an image input means, such as image sensors.
Japanese Patent Laid Open No. 58-176638 discloses a semi-automatic image extraction masking method as follows. First, an approximate outline of an objective constituent image is determined. Secondly, a plurality of sections, each of which is called a partial area hereinafter, including the approximate outline in a group are determined. Thirdly, a voltage corresponding to the average density of each of the partial areas (composed of several pixels) is set up as a threshold. Then the conclusive outline of the objective constituent image defined as the border line between the area (pixels) of which a corresponding voltage is higher than the threshold and the area (pixels) of which a corresponding voltage is lower than the threshold of each partial area is detected.
However, it must be noted at this juncture that the above method becomes fatal, for example, in the following case. Assuming that a partial area N' of the average density P comprises four distinct density areas A, B, C, and D sharing border lines a , b and c in between respective adjoining areas as shown in FIG. 1(a); (1) the border line a becomes the conclusive outline when A&gt;P.gtoreq.B, (2) the border line b becomes the conclusive outline when B&gt;P.gtoreq.C, or (3) the border line c becomes the conclusive outline when C&gt;P.gtoreq.D. As is obvious from the above example, the method is not capable of detecting a plurarity of border lines (outlines) at a time; in other words, no belt-shape area having an identical density value such as the area B or C can be detected.
Furthermore in the above method, since the partial area N' is determined according to an approximate outline of an objective constituent image, the partial areas N'.sub.-1, N'.sub.-2, N'.sub.-3 . . . do not always have the same relation to the actual outline thereof as shown in FIG. 1(b). Therefore the average densities (thresholds) of the partial areas N'.sub.-1, N'.sub.-2, N'.sub.-3 . . . normally differ, which results in obtaining a discontinuous outline, concretely the border lines b , a and c become the conclusive outlines for the partial areas N'.sub.-1, N'.sub.-2 and N'.sub.-3 respectively in the case of FIG. 1(b).
Of course, data of such a discontinuous outline are put to a correction process; however, the correction process is not always performed perfectly.